Customarily, feedthroughs of niobium are used (as described in the German Patent Specification DE-PS 1 471 379). These feedthroughs, however, are only relatively suitable for lamps having long lives and good color rendition, since, especially when lamps have a metal halide fill, the niobium tube and the ceramic sealing material used for the seal corrode considerably. An improvement is described in the U.S. Pat. No. 4,545,799, Rhodes et al. The niobium tube is tightly sealed in the plug without ceramic sealing material due to the shrinking process of the "green" ceramics during the final sintering. This is readily possible as both materials have approximately the same coefficient of thermal expansion (8.times.10.sup.-6 K.sup.-1).
Feedthroughs made from other metals have also been tested.
A feedthrough is known from the U.S. Pat. No. 3,531,853, Klomp, which has a surface of platinum, iron, nickel or cobalt and a core of an alloy that is adapted to the ceramics. The feedthrough may have a conical shape and may be joined to the plug by the use of a ceramic internal support member--with both plug and internal support member also having conical shape--by axial pressing at a defined pressure and in a defined gaseous atmosphere.
Discharge lamps are known from the U.S. Pat. Nos. 4,011,480, Jacobs et al, and 4,160,930, Driessen et al, in which the tubular current feedthrough consists of tungsten, molybdenum or rhenium, with the tube being supported in the interior thereof by a ceramic cylinder having straight, axially aligned walls. The cylinder may be solid or hollow; in the latter case the bore serves as the exhaust plug and is subsequently closed. The seal between the feedthrough and the internally and externally abutting ceramic parts, which have both been finally sintered earlier at a temperature of 1850.degree. C., however, is still carried out by means of a ceramic sealing material so that the susceptibility to corrosion of these lamps has indeed been reduced but does not yet satisfy the requirements for the use in lamps having metal halide fills. In spite of great efforts it has not been possible hitherto to develop a corrosion resistant ceramic sealing material.